1. Field of the Invention
The field of the invention is that of surface acoustic wave devices, especially those used as filters in mobile telephony.
2. Discussion of the Background
To carry out filter and resonator type functions with surface acoustic wave devices, it is important to set up transducer and reflector functions that are as efficient as possible.
Typically, a surface acoustic wave resonator consists of a surface acoustic wave transducer between two arrays of reflective electrodes positioned appropriately so as to trap the acoustic energy and thus create a resonant cavity for which a low reflection loss rate is sought. This can be obtained with lots of electrodes. Indeed, the effective coefficient of reflection of a single electrode is only a few percent. Thus, these arrays are constituted by about a hundred electrodes, or even more, uniformly spaced out at a rate of two electrodes per wavelength. There are essentially two reasons for the weakness of reflection of an electrode: since the electrode is on the surface, it does not react strongly with the acoustic wave whose energy is scattered in the substrate (to substantially increase this interaction would require electrodes of a fineness that cannot be achieved in practice). Furthermore, if the coefficient of reflection of a single electrode is very high, then substantial energy is dispersed in the form of bulk waves and therefore lost for the entire resonator.
To increase the efficiency of the arrays and therefore the performance characteristics of the resonators, it is particularly advantageous to use an acoustic energy guide, namely to use a fine layer of piezoelectric material in which the acoustic wave can be confined.
Surface acoustic wave transducers are increasingly making use of internal reflections to carry out one-directional acoustic transmission. Therefore, this type of transducer benefits in the same way from an increase in reflection efficiency. Furthermore, if the acoustic energy is guided in a thin layer of piezoelectric material, the piezoelectric coupling coefficient is more efficient.
This is why the invention proposes a device using surface acoustic waves, guided in a thin layer of piezoelectric material bonded to a carrier substrate so as to provide for the confinement of the acoustic waves in the piezoelectric material.
More specifically, an object of the invention is a surface acoustic wave device comprising means to create surface acoustic waves and a thin layer of piezoelectric material in which the acoustic waves are guided, characterized in that the device comprises a carrier substrate and a layer of molecular bonder by which the thin layer of piezoelectric material is bonded to the carrier substrate.
The carrier material may be of the glass, sapphire, silicon or gallium arsenide type. The piezoelectric material may be of the quartz, lithium niobate or lithium tantalate type and the molecular bonder may be of the silica type. According to one variant of the invention, the surface acoustic wave device may include a metal layer between the layer of molecular bonder and the layer of piezoelectric material.
Yet another object of the invention is a first method for the manufacture of an acoustic wave device comprising a thin layer of piezoelectric material, a layer of molecular bonder and a carrier substrate.
More specifically, this method is characterized by the following steps:
the deposition of a layer of molecular bonder on a first face of a piezoelectric material substrate,
the joining of a carrier substrate and of the unit formed by the layer of molecular bonder and the piezoelectric material substrate under a wet atmosphere so as to carry out a hydrophilic bonding operation,
the reduction of the thickness of the piezoelectric substrate by a mechanical, chemical or ion type of method so as to define the thin layer of piezoelectric material.
Advantageously, the method may comprise the following steps:
the implantation of ions in the substrate of the piezoelectric material at a depth d with respect to a first face of a substrate of piezoelectric material;
the fast heating of the assembly formed by the layer of molecular bonder and the piezoelectric material substrate so as to crack the piezoelectric material substrate at the depth of ion implantation and define the thin layer of piezoelectric material;
the polishing of the thin layer of piezoelectric material.